1. Field of the Invention
The present invention relates to viewfinders for use in lens-shutter (leaf-shutter) cameras, single-lens reflex cameras or the like.
2. Description of Related Art
In recent years, there have been proposed a great number of lens-shutter cameras in which the size of a photographic optical system thereof has been reduced to a compact form. Further, according to the reduction of the size of the photographic optical system, there is a growing demand for an external viewfinder mounted on the camera, being small in size and having an excellent optical performance.
Generally, to achieve such a compact viewfinder, it is required that the number of constituent lenses thereof is reduced and, at the same time, the refractive power of each lens unit is strengthened. However, this approach causes production of various aberrations. As a result, an optical system of high performance becomes difficult to attain.
Further, the recent trend is that ever-increasing numbers of the optical parts of the external viewfinder for lens-shutter cameras are manufactured by using plastic materials, in consideration of lowering the cost and of improving the productivity. In such a situation, it becomes important, despite no freedom of choosing glass materials, to attain improvements of the performance and of the compact form of the optical system without sacrificing the cost and productivity.
In consideration of the optical performance of the viewfinder, what determines whether the perception is good or bad when looking through the viewfinder depends largely on whether or not there are left small residual chromatic aberrations. If the amount of residual chromatic aberrations is large, color confusion appears at the contours of an object being observed, and flare components are produced, giving no good comfortableness to the observer.
Particularly, for a wide-angle viewfinder to be realized, the lateral chromatic aberration increases with an increase of the field angle. In the situation that, as described above, emphasis is laid on the cost and productivity, because the availability of optical materials is limited and the number of constituent lenses is restricted to a smaller value, a correction for the lateral chromatic aberration is difficult to carry out.
In general, on the premise that ordinary refracting optical elements are made use of, improvements of the optical performance of the real-image viewfinders are carried out by pairing up lens components of positive and negative refractive powers. In order to further improve the correction of all aberrations in good balance, it is unavoidable to increase the number of lens elements in the viewfinder.
Further, in order to reduce the number of lens elements while keeping good optical performance, another effective means is to introduce an aspheric surface. However, as far as the chromatic aberrations are concerned, because the correction for the chromatic aberrations depends on the difference in dispersion index between the materials of the paired lens elements of positive and negative powers, no valuable correcting effect is expected from the use of the aspheric surface.
On account of such a situation, there have been made previous proposals that, without relying on the dispersion characteristics of the optical materials, the amount of generation of chromatic aberrations can be controlled by making use of a diffraction optical element in the optical system, as disclosed in, for example, Japanese Laid-Open Patent Applications No. Hei 6-324262 and No. Hei 9-127322.